Control valve assemblies are commonly used in process control systems. The control valve assembly is typically part of a feedback loop including a process controller and a sensor. The process controller establishes a set point for the process, and the sensor senses a parameter that is dependent on the flow rate of the control valve. When the sensed parameter differs from a desired value, the process controller provides a control signal to the control valve assembly to change the flow rate and thereby reduce deviations from the desired parameter value.
A control valve assembly includes a control valve, a valve actuator and a valve positioner. The control valve may have a variety of configurations, including but not limited to rotary valves, butterfly valves and globe valves. The valve positioner receives an electrical control signal from the process controller and provides an actuator signal to the valve actuator. The positioner may receive an analog signal or a digital signal from the process controller. The valve actuator mechanically controls the valve position. Pneumatic actuators may control valve position in response to controlled air pressure. The air pressure, in turn, may be controlled by a solenoid valve or a piezoelectric device. In a digital positioner, the pneumatic actuator is controlled by air pulses at a prescribed pulse rate. Current-to-pneumatic actuators may also be utilized.
The operating characteristics of the control valve assembly are an important factor in the overall performance of the process control system. Generally, the control valve should provide relatively uniform performance over a range of operating conditions and should not adversely affect the stability of the process control system. One parameter of importance is the gain of the control valve assembly, defined as the change in flow rate for a given change in the control signal. Preferably the gain should be constant or nearly constant over the operating range of the control valve. However, different types of control valves exhibit different, often non-linear changes in flow rate as a function of change in actuator position. It is known to provide a valve positioner having a non-linear characteristic which compensates for the non-linear characteristic of the control valve and provides a linear overall relation between flow rate and control signal.
The gain of the control valve assembly is a static characteristic. Dynamic behavior must also be taken into consideration. Typically, prior art control valves have been operated at constant speed, often resulting in a non-linear change in flow rate as a function of time. For example, the flow rate may change rapidly near the low end of the control signal range but may change slowly near the upper end of the control signal range, despite the fact that the actuator speed is constant. Such characteristics may adversely affect the performance of the process control system. For example, the stability of the process control system may be adversely affected if the control valve assembly reacts slowly under some conditions and rapidly under other conditions.
The control valve assembly should react quickly to control signals which require a change in control valve flow rate. Thus, as described above, the positioner typically operates the actuator at or near maximum speed. To avoid overshoot of the desired flow rate, the speed is typically reduced as the valve approaches the desired flow rate. In a digital positioner, this is accomplished by reducing the pulse rate of the air pulses supplied to the pneumatic actuator. Despite this approach, overshoot may occur. For some values of phase shift in the process control feedback loop, unstable operation may result. U.S. Pat. No. 3,906,196 issued Sep. 16, 1975 to Spitz discloses a feedback system wherein a control signal is a non-linear function of an error signal. The non-linear function is symmetrical with respect to error polarity.
Accordingly, it is desirable to provide control valve assemblies wherein one or more of the above drawbacks are overcome.